1. Field of the Invention
This invention relates to an ALC (automatic level control) circuit in a low frequency amplifier circuit.
2. Background of the Invention
When, in a low frequency amplifier, its input level is excessively high, it is necessary to decrease the level at a suitable rate to prevent the saturation of its output.
An example of a conventional ALC circuit will be described with reference to FIG. 4. In FIG. 4, an input terminal 1 is connected through a voltage dividing resistor R.sub.3 to an output terminal 12. The output produced at the terminal 12 is also supplied through an output detecting resistor R.sub.4 and an output detecting variable resistor VR.sub.5 to an amplifier section 6, where it is amplified. The output of the amplifier section 6 is applied through a time constant circuit 7 made up of an R-C (resistor and capacitor) series circuit and through a rectifying and smoothing circuit 8 to the base of a transistor TR.sub.10 Darlington-connected with a transistor TR.sub.11. The collector of the transistor TR.sub.10 is connected to a positive power source +B and its emitter is connected to the base of the transistor TR.sub.11. The collector of the transistor TR.sub.11 is connected to an output line, i.e., the output terminal 12. That is, the ALC circuit is so designed as to operate as a variable impedance to control the attenuation according to the input level.
The operation of the conventional ALC circuit thus organized will now be described. When the input level is low, a control signal produced from the output line is also low in level and therefore the transistor TR.sub.10 is maintained non-conductive (off) and the transistor TR.sub.11 is also non-conductive, i.e, the transistor TR.sub.11 has a high impedance between its emitter and collector. Accordingly, the output provided on the output line will not be decreased by the transistor TR.sub.11. That is, the output level is not decreased in this case.
When, on the other hand, the input level is increased to render the transistor TR.sub.10 conductive (on), the transistor TR.sub.11 is also rendered conductive by a current which is supplied from the positive power source according to the base-emitter current of the transistor TR.sub.10. That is, the transistor TR.sub.11 assumes a low impedance between its emitter and collector. Accordingly, the output provided on the output line is decreased by the transistor TR.sub.11. That is, the output level is decreased. FIG. 5 shows one example of frequency v. gain characteristics in this operation, in which the ordinate axis represents overall gain defined by dividing an output level by an input level. When the input level is low, the amount of control is small whereas when the input level is high, the amount of control is large, yet providing substantially flat frequency characteristic curves.
In this conventional ALC circuit, because of the circuitry described above, the level attenuation occurs over all the frequency bands concerned. That is, the operation of the ALC circuit makes a person feel the high frequency level variation which is offensive to the ear, thus causing a so-called "breezing phenomenon".
When an excessively large input is applied, especially a low frequency band must be taken into consideration because of the following reason. In general, the ALC operation is often required when a high frequency signal is superposed on a low frequency signal. Since, in terms of acoustic power, a high frequency signal is lower in level than a lower frequency signal, it is unnecessary to perform an ALC operation for all the frequency bands.
Furthermore, the ALC control carried out for each channel suffers from the following difficulty. In the case where two channels, i.e., right and left channels in a stereophonic mode, have different high frequency component attenuation from each other, high frequency components in particular, having significant directivity, adversely affect the sound image localization, thus making a person uncomfortable.